MTJ device performance by controlling device shape

ABSTRACT

A layered thin film device, such as a MTJ (Magnetic Tunnel Junction) device can be customized in shape by sequentially forming its successive layers over a symmetrically curved electrode. By initially shaping the electrode to have a concave or convex surface, the sequentially formed layers conform to that shape and acquire it and are subject to stresses that cause various crystal defects to migrate away from the axis of symmetry, leaving the region immediately surrounding the axis of symmetry relatively defect free. The resulting stack can then be patterned to leave only the region that is relatively defect free.

BACKGROUND 1. Technical Field

This disclosure relates generally to magnetic memory devices,specifically to the effect of the shape of a device on its performance.

2. Description of the Related Art

The crystalline property of the structure of a magnetic memory device,specifically that of the ferromagnetic/MgO tunneling barrier interfaceof a MTJ (magnetic tunneling junction) device, plays a very criticalrole in the device performance. In particular, the strain in thestructure can significantly alter the properties of the device, forreasons to be discussed below.

Magnetic memory devices comprise a stack of layers in which twoferromagnetic layers, typically referred to as a reference layer and afree layer, are separated by a thin, non-magnetic dielectric layer,called a barrier layer. In the classical physics regime, a current ofelectrons going from one ferromagnetic layer to the other would beunable to pass through the barrier layer, which is non-conducting; butaccording to quantum mechanics, electrons can “tunnel” through thebarrier layer if the right conditions exist in terms of the spin of thetunneling electrons and the magnetization directions of the twoferromagnetic layers on either side of the barrier. The conditionsnecessary for the electrons to successfully tunnel also depend on thequality of the interfaces between the barrier layer and theferromagnetic layers. Imperfections in the interfaces make it difficultto achieve high TMR (Tunneling Magneto Resistance) values, which measurethe ability of the electrons to successfully tunnel when the propermagnetization conditions are met. Such imperfections result from latticemismatches between the ferromagnetic layers and the non-magnetic barrierlayer and from defects that occur during crystal growth of materials.These undesirable qualities are associated with strain which, in turn,causes reduction in the TMR values, as has been documented, for example,in “Strain-enhanced tunneling magnetoresistance in MgO magnetic tunneljunctions,” by Li Ming Long et al., Scientific Reports 4, Article number6505 (2014) and also “Tuning the magnetic anisotropy of CoFeB grown onflexible substrates” by Zhang Hao et al. in Chinese Physics B, Volume24, Number 7-077501 (2015)

Given the fact that the shape of a crystalline layer affects thestresses and strains in the layer which, in turn, affects the growth ofdefects in that layer, controlling layer shape should permit acorresponding control of and even a purposeful manipulation of thestresses in an MTJ film stack. Therefore, such a shape controllingprocess should enable improvements in device performance, for example,improving the TMR and the coercivity of the device. More specifically,the new shape of a device, if properly designed and produced, can beexpected to reduce the interface defect concentration and improveinterface lattice epitaxy, all of which will improve device performance.

Attempts to influence device performance by shape control are known inthe prior art, for example, Ahn et al. (U.S. Pat. No. 7,998,758) and Kimet al. (U.S. Pat. No. 9,305,928). However these attempts do not use thesame methods or produce the same effects as will be described herein.

SUMMARY

A first object of the present disclosure is to provide a method ofimproving the performance of a layered MTJ device by controlling itsshape.

A second object of the present disclosure is to provide such a methodthat reduces interfacial defect concentrations and lattice mismatchesand improves lattice epitaxy, thereby creating a measurable improvementin TMR.

A third object of the present disclosure is to provide such a methodthat allows controlling and manipulating the shape of a layered deviceduring its fabrication, in effect customizing its shape.

A fourth object of the present disclosure is to control the stresses ina TMJ layered film stack or similar device structure by controlling itsshape so that the stresses thereby cause crystal defects to migratetowards regions of the structure that can be subsequently removed.

These objects will be achieved through the design and fabrication of apatterned, layered MTJ device during which its shape is controlled insuch a manner that crystal defects such as vacancies, pinholes anddislocations within the stack are moved to an undesirable weak zonewhich can subsequently be removed. The relatively defect-free remainingportion of the device will have a higher coercivity and improved TMRvalues. The new device shape will, therefore, reduce the interfacedefect concentration and improve the interface lattice epitaxy, bothresults leading to improvement of device performance as, for example,measured by the improved TMR values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art MTJ device.

FIG. 2 schematically shows a dome-shaped bottom electrode for the devicein FIG. 1 in accord with the method of the present disclosure.

FIG. 3 schematically shows the effects on the formation of a layered MTJstack when the stack is formed over the dome-shaped electrode of FIG. 2.

FIG. 4 is a schematic illustration showing the effects of patterning thestack of FIG. 3, so that laterally disposed regions of the stackcontaining defects resulting from its formation over a dome-shapedelectrode are eliminated.

FIG. 5 is a schematic illustration of an alternative electrode which isconcave rather than convex as is the dome-shaped electrode.

FIG. 6 is a schematic illustration, analogous to that of FIG. 3, but nowshowing the effects of forming a MTJ stack over the concave electrode ofFIG. 5.

FIG. 7 is a schematic illustration showing the effects of patterning thestack of FIG. 6, so that laterally disposed regions of the stackcontaining defects resulting from its formation over a concave electrodeare eliminated.

FIG. 8 is a process flow diagram showing the sequence of steps that leadto the structure illustrated in FIG. 1.

FIG. 9 is a process flow diagram showing the sequence of steps that leadto the structure illustrated in FIG. 4.

FIG. 10 is a process flow diagram showing the sequence of steps thatlead to the structure illustrated in FIG. 7.

DETAILED DESCRIPTION

Referring first to FIG. 1, there is shown a standard patterned MTJ(Magnetic Tunneling Junction) layered device such as is commonly used ina random access memory (RAM) array. Although a specific substrate is notnecessary to define the device, for specificity we will consider thedevice to be formed on a functional substrate, such as a CMOS substrate,that may already include circuitry and conducting vias to access variouselements of the devices in a RAM array.

The device is formed by first providing a CMOS substrate 10 on which toform it. A blanket bottom electrode (BE) 20 is deposited on the CMOSsubstrate so that it properly contacts the vias in that substrate. Thereis then deposited on top of the bottom electrode 20, in succession, aseries of horizontal layers that will comprise the MTJ stack. Thoselayers include, a seed layer 30, a pinned magnetic layer 40, a tunnelbarrier layer 50, a free magnetic layer 60 and capping layer 70. A hardmask 80 is deposited on the capping layer. The hard mask is thenpatterned by a standard photolithographic process (not shown) and thefilm stack is then patterned using the patterned hard mask as a guide.Note that the bottom electrode is also shown as patterned and it issurrounded laterally by dielectric fill material 90.

The process steps (801-807) leading to the formation of the typicalprior art MTJ as shown in FIG. 1 and described in detail above, can besummarized in the flow diagram 800 shown in FIG. 8.

We will now describe the method of the present disclosure as illustratedwith reference to FIG. 2-FIG. 4. We note at the outset that the methodwill be described with reference to an MTJ device, but it is applicableto any layered, crystalline structure that can be expected to accumulatevarious types of defects as it is grown.

Like the prior-art method described by the process flow chart in FIG. 1,above, the method of the present disclosure also begins with a CMOSsubstrate 10 (although other substrates are also possible) and, on thissubstrate a bottom electrode layer 20 formed of conducting material isdeposited. However, the present method deviates from the prior-artmethod above by patterning and etching the electrode to form a “dome”(i.e., a symmetrical convex) shape 25, as is shown in FIG. 2. Note thatthe “dome” or convex shape, is here axially (about a horizontal axis)symmetric, so it is more correctly a cylinder (or a semi-circularcylinder if the convex shape has a semi-circular cross-section).

There are several methods by which the electrode can be formed with asurface having a convex, cylindrical shape, or indeed, by which anelectrode can be formed with various surface shapes other than a convexshape, such as a concave shape to be discussed below. For example, aninitial etch can be performed on a layer of electrode material to createa solid rectangular prism and then a partial etch (or series of partialetches) can be performed that successively removes corners of theelectrode to give it the rounded surface shape. Alternatively, a sputteretch can be performed to produce a rounded surface shape. To form aconcave surface, isotropic physical etching can be performed.

Referring next to schematic FIG. 3, there is shown the result of asequential deposition process in which there is first deposited on theconvex cylindrical electrode 25, a seed layer 35, which is followed by apinned layer 45, a barrier layer 55, a free layer 65, a capping layer 75and a hard mask layer 85. These layers will all conform to the shape ofthe electrode, by each layer conforming, in sequence, to the shapedlayers beneath it. For definiteness we note that the above process stepscan be applied to a typical MTJ structure having a seed layer of Ta, Ru,W or NiCr with a thickness range of between approximately 50-500 A(angstroms), ferromagnetic layers for pinned and free layers formed ofCoFeB with a thickness range between approximately 10-100 A, a tunnelingbarrier layer of AlOx or MgO of thickness between approximately 5-50 Aand a capping layer of Ta, W or Mg with a thickness range betweenapproximately 10-100 A. The convex electrode can have a thickness ofbetween approximately 100-1000 nm and have a width between approximately50-500 nm.

Due to the underlying convex shape, as successive layers are formed anddevelop their crystalline structure, the inevitable defects produced bythe crystal growth mechanism, such as pinholes, vacancies anddislocations, will move away (migrate) from the top of the convexity ofeach successively formed layer and accumulate at the corners where theunderlying curved shape of the electrode becomes a horizontal layer.This is what is called the “weak” region of the structure (see theregion surrounded by an ellipse 90).

The region of the stack containing a small region to either side of thepeak of the convex structure will be relatively defect free. Bycontrolling the slope (curvature) of the convexity, the film strain canbe manipulated, and the lattice mismatch and interfacial defectconcentration can be reduced within a desired region symmetricallydisposed to either side of the peak of the convex structure. Note thatthe width of the convexity that can be considered relatively defect freecan cover most of the total width of the curved region with exception ofthe weak regions 90 at the corners. In most cases, more than half of thewidth of the convexity, centered about its highest point can beconsidered defect free. For example, an electrode having a base width of200 nm will produce a defect free device shape of at least 100 nm.

After the convex structure is patterned to produce the vertical stack,which is actually a cylindrical slice, as shown in schematic FIG. 4, thegaps to either side 110 are filled with a gap-filling dielectricmaterial (not shown) and the upper surfaces of the stack and itssurrounding dielectric are planarized and polished to remove the masklayer in preparation for further process integration. Note that FIG. 9is a process flow 900 that briefly describes the sequence of steps(901-906) that lead to the structure in FIG. 4.

Referring next to schematic FIG. 5, there is shown an alternativeembodiment of the present process where the bottom electrode 27 isshaped to have a concave upper surface and is otherwise axiallysymmetric.

Referring next to FIG. 6, there is shown that the film stack of the MTJ(or any layered crystalline construction) can be deposited over theconcave bottom electrode 27 in the same manner as it was deposited overthe previously described convex electrode. FIG. 6, shows schematicallythe result of a sequential deposition process in which there is firstdeposited on the concave electrode 27 a seed layer 37, which is followedby a pinned layer 47, a barrier layer 57, a free layer 67, a cappinglayer 77 and a hard mask layer 87. These layers will all try to conformto the concave shape of the electrode, by each layer conforming, insequence, to the concave-shaped layers beneath it. Note that thematerial layers can be formed of the same materials and dimensions asdescribed above for the convex structure.

Due to the underlying concave shape, as successive layers are formed anddevelop their crystalline structure, the inevitable defects produced bythe crystal growth mechanism, such as pinholes, vacancies anddislocations, will move away (migrate) from the bottom point of theconcavity of each layer and accumulate at the upper corners where theconcavity, discontinuously, merges with the horizontal layer of thesubstrate 10. The region of the stack containing a small region toeither side of the concave minimum will be relatively defect free. Thisregion surrounding the discontinuity, called a “weak” region, is shownenclosed in an elliptical region 100. It is at this weak region thatdefects will accumulate. By controlling the shape of the electrode 27the film strain in the sequentially deposited layers can be manipulated,and the lattice mismatch and interfacial defect concentration can bereduced in a specific region.

After the concave-shaped structure is patterned to produce the verticalstack that now contains the defect-free bottom of the concavity, asshown in schematic FIG. 7, the gaps to either side 110 are filled with agap-filling dielectric material and the upper surfaces of the stack andits surrounding dielectric are planarized and polished in preparationfor further process integration (not shown).

Note that FIG. 10 is a process flow 1000 that briefly describes thesequence of steps (1001-1006) that lead to the structure in FIG. 7.

As is finally understood by a person skilled in the art, the detaileddescription given above is illustrative of the present disclosure ratherthan limiting of the present disclosure. Revisions and modifications maybe made to methods, materials, structures and dimensions employed informing and providing a layered crystalline magnetic device such as anMTJ device, whose layer strain is controlled during layer formation toreduce crystal defects and thereby to improve device performance, whilestill forming and providing such a structure and its method of formationin accord with the spirit and scope of the present invention as definedby the appended claims.

What is claimed is:
 1. A method of controlling effects of layer strainwhile forming a magnetic thin film device, comprising: providing asubstrate having a flat upper surface; forming on the substrate anelectrode layer having a flat upper surface; symmetrically shaping saidflat upper surface of said electrode layer to form a curved surfacehaving an axis of symmetry; sequentially depositing a layered devicestructure: over the symmetrically shaped upper surface of saidelectrode, whereby each layer in said layered device structure acquiresa shape corresponding to said symmetrically shaped upper surface of saidelectrode with laterally extending portions over the flat upper surfaceof said substrate then using a hard mask layer as a guide, removingsymmetric portions of said layered device structure that are laterallydisposed to each side of said axis of symmetry, leaving thereby aremaining portion of said layered device structure, wherein said removedportions contain weak regions where said curved surfaces meet saidsurrounding flat surfaces; wherein layers in said removed portionscontain vacancies, crystal defects, pinholes and dislocations that havemigrated to said weak regions and collected therein during formation ofsaid layered device structure and, wherein a remaining portion of saidlayered device structure is relatively free of vacancies, crystaldefects, pinholes and dislocations.
 2. The method of claim 1 furtherincluding surrounding said remaining portion of said layered devicestructure by a dielectric fill layer and planarizing said dielectricsurrounded remaining portion in preparation for further processintegration.
 3. The method of claim 1 wherein said vacancies, crystaldefects, pinholes and dislocations migrate to said laterally disposedregions as a result of strains induced in layers due to their curvature.4. The method of claim 1 wherein said curved region is convex.
 5. Themethod of claim 1 wherein said curved region is concave.
 6. The methodof claim 1 wherein said layered device structure comprises: a bottomelectrode having a surface that is symmetrically curved about an axis;upon which is sequentially formed: a seed layer; a ferromagnetic pinninglayer; a tunneling junction layer; a ferromagnetic free layer; cappinglayer; and a hard mask layer.
 7. The method of claim 6 wherein said seedlayer is a layer of Ta, Ru, W or NiCr with a thickness range of betweenapproximately 50-500 A (angstroms); said ferromagnetic free layers andpinning layers are formed of CoFeB with a thickness range betweenapproximately 10-100 A; said tunneling barrier layer is formed of AlOxor MgO of thickness between approximately 5-50 A and said capping layeris formed of Ta, W or Mg with a thickness range between approximately10-100 A.
 8. The method of claim 6 wherein said electrode has athickness of between approximately 100-1000 nm and has a width betweenapproximately 50-500 nm.
 9. The method of claim 8 wherein two regions,symmetrically disposed about the axis of symmetry of said layered devicestructure, are removed; whereby a remaining portion of said layereddevice structure has a width between approximately 50-500 nm.
 10. Ashape-controlled magnetic layered device having strain controlled anddefect-free crystalline layers, comprising: a bottom electrode, havingan initially axially symmetric curved surface; a sequence of layersformed on said curved electrode; wherein said sequence includes layershaving a crystalline structure and wherein strain in said layers iscontrolled by said layers conforming to said curved bottom electrodesurface during formation; wherein each of said sequence of layers isdefect-free within a first region symmetrically disposed about the axisof symmetry of said electrode, but wherein defects have accumulatedwithin a second region laterally disposed beyond said first region;wherein said second region has been removed and said remaining firstregion is defect free.
 11. The device of claim 10 wherein said initialsurface shape of said bottom electrode is concave.
 12. Ashape-controlled magnetic layered device having strain-controlled anddefect-free crystalline layers, comprising: a bottom electrode, havingan initially axially symmetric convex surface; a sequence of layersformed on said convex electrode; wherein said sequence includes layershaving a crystalline structure and wherein strain in said layers iscontrolled by said layers conforming to said convex bottom electrodesurface during formation; wherein each of said sequence of layers isdefect-free within a first region symmetrically disposed about the axisof symmetry of said electrode, but wherein defects have accumulatedwithin a second region laterally disposed beyond said first region;wherein said second region has been removed and said remaining firstregion is defect free and wherein a dielectric fill layer is thereupondeposited to either side of said first region and the upper surfaces areplanarized and polished and prepared for further integration.
 13. Ashape-controlled magnetic layered device having strain-controlled anddefect-free crystalline layers, comprising: a bottom electrode, havingan initially axially symmetric curved surface; a sequence of layersformed on said curved electrode; wherein said sequence includes layershaving a crystalline structure and wherein strain in said layers iscontrolled by said layers conforming to said curved bottom electrodesurface during formation; wherein said layers further comprise: a seedlayer; a ferromagnetic pinned layer; a tunneling barrier layer; aferromagnetic free layer; and a capping layer; and wherein each of saidsequence of layers is defect-free within a first region symmetricallydisposed about the axis of symmetry of said electrode, but whereindefects have accumulated within a second region laterally disposedbeyond said first region; wherein said second region has been removedand said remaining first region is defect free.
 14. The device of claim13 wherein said bottom electrode is a layer of conducting material, saidseed layer is a layer of Ta, Ru, W or NiCr with a thickness range ofbetween approximately 50-500 A (angstroms); said ferromagnetic freelayers and pinning layers are formed of CoFeB with a thickness rangebetween approximately 10-100 A; said tunneling barrier layer is formedof AlOx or MgO of thickness between approximately 5-50 A and saidcapping layer is formed of Ta, W or Mg with a thickness range betweenapproximately 10-100 A.
 15. The device of claim 14 wherein said bottomelectrode is initially formed with a convex axially symmetric surfaceshape.
 16. The device of claim 14 wherein said bottom electrode isinitially formed with a concave axially symmetric surface shape.